Add extensible visitor support mutable partial variants

[soareschen]

Summary

This PR adds support for using extensible visitors with mutable partial variants. With this, we can get a &mut reference of an enum, and perform visitor dispatch with mutable access to each variant.

In order to support this, a new HasExtractorMut trait has been added, to extract mutable variant fields. A new MapTypeRef trait is also added to map a T into either &'a T or &'a mut T depending on whether IsRef or IsMut is used.

Example

Consider a trait like:

pub trait CanScale {
    fn scale(&mut self, factor: f64);
}

We can now use a new visitor dispatcher MatchFirstWithValueHandlersMut to dispatch on a mutable reference of a shape:

impl CanScale for Shape {
    fn scale(&mut self, factor: f64) {
        MatchFirstWithValueHandlersMut::<Scale>::compute(&(), NoCode, (self, factor))
    }
}